Ultra-thin fibers (so-called nanofibers, i.e., fibers with a fiber diameter of predominantly (more than 99% of the total number of fibers) in the range of less than 500 nm) are nowadays produced, aside from known methods such as meltblowing or island-in-the-sea, primarily by the so-called electrospinning method. Electrospinning (also referred to as electrostatic spinning) is a multi-purpose method for producing, from solutions and a melt, primarily of polymers, continuous fibers with diameters of a few micrometers up to a few nanometers. The method is versatile because almost all soluble and meltable polymers can be employed; the polymers moreover can be furnished with various additives, from simple soot particles to complex species such as enzymes, viruses, and bacteria; and, of course, chemical modifications are also possible.
Actually, electrospinning is not a fiber spinning method but instead is a method for coating with microfiber and nanofiber nonwovens. By electrospinning, almost any solid and liquid substrates can be coated with a thin layer of polymer fiber nonwovens, typically having a weight per surface area of less than 1 g/m2. Such electrospun polymer fiber nonwovens are extremely delicate and actually can essentially be used only when applied to substrates. By a significant increase of the productivity of the electrospinning method, also self-supporting electrospun tissues are now accessible that have significantly higher weights per surface area (up to 200 g/m2) and therefore can no longer be considered a coating.
FIGS. 1a and 1b show schematically the configuration of an electrospinning experiment for fiber production. In today's electrospinning apparatus a polymer solution or melt 22 is sprayed from a thin steel wire 5 with a diameter of approximately 0.2 mm thickness. The wires are arranged on a cylinder and are immersed at regular intervals by a circular movement into the spinning solution so that they are coated with solution or melt. Since the spinning solution container 21 is at high voltage, this causes the solution to be sprayed off from the wires 5. The wire electrodes that are used in the context of the invention are disclosed, for example, in WO 2008/98526 and WO 2008/028428.
The applied voltage effects a conical deformation of the droplet in the direction of the counterelectrode. Along the path to the counterelectrode, the solvent contained in the spinning solution will evaporate (or the melt will solidify) and on the counterelectrode solid fibers with diameters of several micrometers down to a few nanometers are deposited at high speed.
As already mentioned, for the electrospinning method almost all soluble and meltable polymers can be used. Polytetrafluoroethylene (PTFE) is a high-temperature polymer that is characterized by an excellent resistance with respect to chemicals and environmental effects and moreover has a crystallite melting point of 327 degrees Celsius. However, it hardly dissolves in solvents because of its chemical resistance. Therefore, no classical spinning solution for the electrospinning process can be provided in order to produce fine fibers in a nanometer and/or sub-micrometer range. PTFE fibers produced on the basis of a melt electrospinning process are also not been available at this time. The reason is the relatively high melting range and the decomposition tendency of the melt with poisonous hydrogen fluoride being cleaved off. Moreover, PTFE has the tendency to creep. Generally, the aforementioned methods for processing PTFE are very complex and their usability for the manufacture of relatively simple pressed and sintered parts from powder and pastes is limited.
Still, PTFE with its excellent electret properties and its distinct hydrophobic properties is a very interesting material with respect to filtration purposes. Particularly negative charges, for example, caused by corona discharge, can be stored excellently on the PTFE surfaces. Discharge of PTFE by moisture occurs only very slowly because the material, as already mentioned, is highly hydrophobic.
Since PTFE practically cannot be spun to very thin fibers and fiber-like structures can be obtained only by stretching of PTFE films, already known technical solutions are based on fiber-like membranes. The latter can be produced with high technical expenditure from PTFE and subsequently can be provided optionally with additional fibers of other polymer materials. For example, EP 1 878 482 discloses a filter medium with a porous PTFE membrane, an air-permeable support element, and a web layer which is produced by electrospinning from polymer fibers.
Known in the prior art are also microporous PTFE membranes that are used in pieces of clothing.
Another strategy is to use other fluoropolymers and therefore to “imitate” the desired properties of PTFE. For example, WO 2009/018463 discloses various blends of different fluoropolymers that are spun from an acetone solution to a fine fiber layer and subsequently are crosslinked by high-energy electron beams. This method is however very complex and the fine fiber layer obtained thereby of fluoropolymers can be used subsequently only for the described application purpose of separation of water from liquid hydrocarbon mixtures (for example, diesel fuel).
U.S. 2010/0193999 discloses an improved method for producing a PTFE fiber mat by electrospinning of PTFE dispersions with a viscosity of at least 50,000 cP. In this connection, the electrostatically charged dispersion is collected on a target and forms thereat a pre-product from which, by heating, the solvent is removed and, in this way, the PTFE fiber mat is formed. The document contains no information as regards the diameter of the produced PTFE fibers. Further information with respect to processing and electrospinning of PTFE from an aqueous dispersion or other dispersions are provided in U.S. Pat. Nos. 4,323,525; 4,127,706; and 4,044,404. The fiber diameters of PTFE fibers disclosed therein are in the range of 0.1 to 25 micrometers.